Improved coal gasification

ABSTRACT

A two-zone gasifier is used to produce syngas from coal fines, oxygen and steam. The two-zone gasifier has a top fluidized gasification zone and a bottom entrained flow gasification zone connected by a Venturi throat. The syngas production will occur in both zones with the syngas being collected from the top of the gasifier.

BACKGROUND OF THE INVENTION

Coal gasification technologies are mainly classified into three types:fixed bed gasifiers such as those made available from Lurgi, fluidizedbed gasifiers such as those from UCAS and entrained flow gasifiers likethose from Shell and General Electric, The fixed bed gasifiers havedisadvantages in terms of low throughput of single unit and costlysyngas and water treatment systems. The fluidized bed gasifiers haveproblems with dry bottom ash discharging, high fly ash carried over andlow carbon conversions, For the entrained flow gasifiers, the majordefects are narrow range of suitable types of coal feedstock and theparticle size of the coal fed to the gasifier must be kept below 75micron. Additionally, large amounts of steam such as in TPRI gasifiersor other types of slurry gasifiers or cold syngas gas such as in Shellgasifiers must be injected for cooling purposes to ensure thetemperature of the raw syngas outlet of the gasifier remains below 950to 1000° C. due to the requirements of the downstream waste heat boiler.This can dramatically reduce the thermal efficiency of the gasificationprocess.

The invention seeks to overcome these limitations by combining normalfluidized bed and entrained flow coal gasification technologies toimprove their operation, efficiencies and reduce costs.

SUMMARY OF THE INVENTION

In one embodiment of the invention, there is disclosed a gasifier forproducing syngas comprising an upper fluidized gasification zone influid communication through a Venturi throat with a lower entrained flowgasification zone.

The gasifier has refractory lining walls in the upper fluidizedgasification zone and water membrane was in the lower entrained flowgasification zone. The two gasification zones have pipes and openings toavow for the injecting of materials for forming syngas, namely coal,coal fines and other reactants such as oxygen and steam.

The two gasification zones are connected by a Venturi throat which has adiameter that is narrower than both gasification zones. The entrainedflow gasification zone has an opening at the bottom to avow for theremoval of liquid slag while the fluidized gasification zone has anopening at the top to allow for recovery of the syngas produced as wellas fines and other product gases produced.

In another embodiment of the invention, there is disclosed a method forproducing syngas comprising the steps:

a) Feeding coal fines, oxygen and steam simultaneously to a fluidizedgasification zone and an entrained flow gasification zone;b) Reacting the coal fines in the presence of oxygen and steam to formsyngas;c) Feeding syngas from the entrained gasification zone to the fluidizedgasification zone; andd) Recovering syngas from the fluidized gasification zone.

The coal fines that are fed to the fluidized gasification zone arelarger than those fed to the other zone and are greater than 75 micronin size. The coal fines fed to the entrained flow gasification zone areless than 75 micron in size.

The fluidized gasification zone is at a temperature of about 1000° C. orbelow the coal ash deformation temperature, while the entrained flowgasification zone is at a temperature of about 1500° C. or above thecoal ash fluid temperature.

The syngas that is produced in both gasification zones is fed through anopening at the top of the fluidized gasification zone at a temperatureof about 900 to 1000° C. to a first stage cyclone which will allow forrecovery of the syngas as well as the recovery of unreacted coal fineswhich can be fed back to the fluidized gasification zone.

Steam is injected into said entrained flow gasification zone to drivethe produced syngas into said fluidized gasification zone.

A second cyclone will capture smaller unreacted coal fines and these arerecycled back to the entrained flow gasification zone.

The gasifier and methods of the invention can be used in a variety ofapplications such as integrated gas combined cycle, coal to chemicals,production of synthetic natural gas, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic of a two-stage coal gasifier according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the figure, the two-stage coal gasifier is shown. Afluidized gasification zone A sits above an entrained flow gasificationzone B being connected by a Venturi throat C. The fluidized gasificationzone has wails that comprise a refractory material D. The entrained flowgasification zone has wail that comprise by a water membrane wall E.

Gasification is occurring in both zones such that coal particles ofvarious particle size distributions are contacted with oxygen and/orsteam. The resulting products include syngas and a measured amount ofcarbon dioxide plus methane,

Coal having a particle size below 75 microns and fines recycled from asecond stage cyclone are injected through line 9 into the entrained flowgasification zone B. Typically the coal is transported in nitrogen orcarbon dioxide. The gasification temperature is controlled at around1500 to 1600° C. above the coal ash fluid temperature. The operatingpressure is about 3.0 to 4.0 MPa.

The steam that is injected into the entrained flow gasification zone Bis used to control the gas temperature and flow rate entering the upperfluidized gasification zone A. The water membrane wall E is essentiallyhigh-pressure tubes containing water which helps cool the entrained flowgasification zone B. Steam is also generated inside the tubes. The tubesare provided with studs that act as anchors for a thin layer of castablerefractory which is typically silicon carbide. During operation of thegasifier, the castable refractory will ideally be covered by a layer ofsolid slag, over which the liquid slag will run to the bottom 10 of theentrained flow gasification zone B.

The hot syngas that is generated enters the fluidized gasification zoneA through a Venturi throat C where the local temperature above theVenturi throat C is maintained at about the coal ash softeningtemperature. The Venturi throat C also functions as a “classifier” inthe sense that agglomerate char particles with higher density could dropinto the section below the Venturi throat C and carry out secondaryreactions thereby becoming melting slag. The melting slag will exit theentrained flow gasification zone B through opening 10 for quenching.

Coal having a larger particle size, above 75 microns and below 10millimeter is fed into the dense phase of the fluidized bed section inthe fluidized gasification zone A through line 1. The fluidizing gas isoxygen and steam, The fines collected from the first stage cyclone (notshown) are recycled and reinjected into the fluidized gasification zoneA through line 2. The fluidizing oxygen and steam are injected throughlines 3, 4, 5 and 6. The raw syngas produced which is primarily hydrogenand carbon monoxide with some methane present as well will exit thefluidized gasification zone A through opening 11 to the first stagecyclone. This syngas reaction temperature is at a temperature of about900 to 1000° C. or below the coal ash deformation temperatures. Thedischarged rate of reacted residues or char is controlled by the Venturithroat C.

The fluidized gasification zone A is typically constructed with a normalrefractory wail which consists of an outer layer of insulating bricks toprotect the outer steel she of the reactor from high temperatures and aninner layer of more compact bricks that can better withstand the hightemperatures and the erosive conditions inside the gasifier.

The cyclones that are used in the methods of the invention connect tothe upper section of the gasifier, fluidized gasification zone throughline 11 and the bottom part of the gasifier, entrained flow gasificationzone B through line 9. The first cyclone captures 98 to 99% of the finescarried over the top of the gasifier through line 11. These finescontain significant amounts of carbon and are recycled to the fluidizedgasification zone A through line 2 for secondary reactions in thegasifier. The second cyclone captures the remainder of fines having asmaller particle size which will lower gasification reactivity. Thesesmaller fines are recycled in to the bottom of the entrained flowgasification zone B through line 9 where they are reacted under moresevere conditions to burnout the extra amount of carbon present in thesmaller fines.

Advantages of the invention include an ash slagging and quench procedureto alleviate the problem of dry ash discharging of fluidized bedgasifiers. The slag can simply be discharged through the bottom 10 ofthe entrained flow gasification zone B. By using a wider range of coalparticle size distribution (PSD) of particles both smaller than 75micron and larger than 75 micron, this will reduce the consumption ofcoal mills as well as allow for a wider range of coal feedstocks thatcan be employed in producing syngas. The carbon conversion of fluidizedgasifiers can be improved by increasing the ash discharging temperature.Further, the thermal efficiency of entrained flow gasifiers can beimproved by eliminating the recycle of cold syngas or extra steam. Alsothe amount of fly ash carried over from the fluidized bed gasifiers isreduced and with it there would be less disposal costs.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the invention.

Having thus described the invention, what we claim is:
 1. A gasifier forproducing syngas comprising an upper fluidized gasification zone influid communication through a Venturi throat with a lower entrained flowgasification zone.
 2. The gasifier as claimed in claim 1 wherein saidupper fluidized gasification zone has refractory lining walls.
 3. Thegasifier as claimed in claim 1 wherein said entrained flow gasificationzone has water membrane walls.
 4. The gasifier as clairned in claim 1wherein said upper fluidized gasification zone has inputs to provide forinjecting materials selected from the group consisting of coal, fines,gas and steam.
 5. The gasifier as claimed in claim 1 wherein saidentrained flow gasification zone has inputs to provide for injectingmaterials selected from the group consisting of coal, fines, gas andsteam.
 6. The gasifier as claimed in claim 1 wherein said Venturi throathas a diameter small than said upper fluidized gasification zone andsaid entrained flow gasification zone.
 7. The gasifier as claimed inclaim 1 wherein said entrained flow gasification zone has an opening atits bottom.
 8. A method for producing syngas comprising the steps: a)Feeding coal fines oxygen and steam simultaneously to fluidizegasification zone and entrained flow gasification zone; b) Reacting saidcoal fines in the presence of oxygen and steam to form syngas; c)Feeding syngas from said entrained gasification zone to said fluidizedgasification zone; and d) Recovering syngas from said fluidizedgasification zone.
 9. The method as claimed in claim 8 wherein saidfluidized gasification zone is at a temperature of about 1000° C. 10.The method as claimed in claim 8 wherein the coal fines fed to saidfluidized gasification zone are greater than 75 micron in size.
 11. Themethod as claimed in claim 8 wherein said fluidized gasification zone isconnected to said entrained flow gasification zone by a Venturi throat.12. The method as claimed in claim 11 wherein said Venturi throat issmaller in diameter than said fluidized gasification zone and saidentrained flow gasification zone.
 13. The method as claimed in claim 8wherein said entrained flow gasification zone is at a temperature ofabout 1500° C., or above the coal ash fluid temperature.
 14. The methodas claimed in claim 8 wherein the coal fines fed to said entrained flowgasification zone are less than 75 micron in size.
 15. The method asclamed in claim 8 wherein liquid slag is fed to a quench from saidentrained flow gasification zone.
 16. The method as claimed in claim 8wherein said syngas is fed to a first stage cyclone.
 17. The method asclaimed in claim 15 wherein said syngas is at a temperature of about 900to 1000° C.
 18. The method as claimed in claim 8 wherein coal fines thatare recycled from the first stage cyclone are fed to said fluidizedgasification zone.
 19. The method as claimed in claim 8 wherein coalfines that are recycled from a second stage cyclone are fed to saidentrained flow gasification zone.
 20. The method as claimed in claim 8wherein steam is injected into said entrained flow gasification zone todrive the produced syngas into said fluidized gasification zone.